Squash Bees Are Pollinating Your Pumpkins and Zucchini

For years, I have felt rather sheepish for never having seen a squash bee. As native bees go, these fetching little stripey, round-faced bees get a lot of press. They’re common and easy to recognize, and they happen to do a very specific job that’s easy to appreciate: They pollinate around 2/3 of the commercially grown squash in the US. When bee enthusiasts are polled for their favorite species, someone always picks the squash bee. So yeah, I felt a little left out.

This female squash bee has been foraging in this male flower (note the pollen on her back legs). (Photo: E. Youngsteadt)

The problem with squash bees (Peponapis pruinosa) is that they get up really early in the morning. They have an unusual relationship with plants in the genus Cucurbita, which includes summer squash, winter squash, zucchini, pumpkins, and many gourds (but not cucumbers). Female squash bees provision their nests exclusively with pollen from Cucurbita flowers and often build their burrows in the soil right under the plants. Even the male bees spend a lot of time at the squash flowers—not just to dine on their plentiful nectar, but also to meet the ladies who come in searching for pollen.

Specializing on squash flowers means waking up at dawn, when those flowers first unfurl. By mid-day, the blossoms crumple, ready to fall off the vine. So by the time I’m out working in the garden, this is usually what I see on a pumpkin vine: Drab remains of flowers, and no bees.

It might have gone on like this indefinitely had we not adopted a cat last winter. Meet Cheddi, a lovey-dovey and very verbal Maine Coon who likes mornings nearly as much as squash bees do. This summer, just when my pumpkins were starting to bloom, Cheddi was in full alarm-clock mode at 6 am: Come on, sleepy people, you are wasting time. You’re not up yet? You should be doing things! Get up! You could come and feed me. Are you awake NOW? It’s morning! Things are happening!

Some days, the silent treatment just doesn’t work. Also, the chickens were making a ruckus. So, still pajama-clad, I shambled out into the dewy dawn past the pristine, glowing-orange pumpkin flowers to let the hens out. And groggily wondered—hey, it’s early, and I have squash flowers. Maybe there are bees.

There were. There was a female, head-first in a flower, her back legs fat with yellow pollen. I felt initiated. And in fact, I’ve seen the bees most mornings since. They’re almost always still out by 8 or 9 am, at which point a few other species—hibiscus bees, green sweat bees, two-spotted long-horned bees, and even honey bees—show up in the flowers, too.

Squash bees are similar in size to honey bees, but have a rounder, more robust build (and forage earlier in the morning). (Photo: E. Youngsteadt)

Each female squash flower starts out with a tiny fruit behind it, but the fruit will only develop if the flower is pollinated. It takes 6 – 10 squash-bee visits to fully pollinate a flower. (Photo: E. Youngsteadt)

From the plant’s point of view (and the gardener’s), these insect visitors are indispensable. Cucurbita’smale and female reproductive parts are housed in separate flowers, and their pollen grains are big and heavy. Bees (or, ok, hand pollination) are the only means to get them from one flower to another. It is no exaggeration to say that without bees, there would be no squash. Not just smaller, uglier ones; none at all.

It takes only 6 to 10 squash-bee visits to fully pollinate a female flower—and squash bees normally get this much done in the first half-hour of foraging. At that point, it’s still barely even full daylight out, and other bees are barely beginning to forage. Although honey bees and bumble bees can also be decent to excellent pollinators for squash, they are superfluous in the presence of the early-rising squash bees.

It works: I have pumpkins. (Photo: E. Youngsteadt)

If you want to encourage these bees in your own garden, there are a few things you should know about them. They are pretty common in suburban areas to begin with—more so than in natural areas, since our cultivated squash are more common than wild gourds in most parts of the country. I only have two pumpkin vines, and the next-door neighbor grows some zucchini, and that seems to be enough to attract the bees.

Because they nest in the ground, often right under the squash plants, these bees are sensitive to things you may do to your garden soil. The female bees usually build their nest cells 6 to 12 inches underground, and the next generation of bees spends most of the year helplessly sealed inside those cells. So tilling a squash bed at any time of year can destroy a lot of bees. A census of 25 squash and pumpkin farms in Virginia found three times more squash bees on no-till farms than on those that tilled.

Other soil treatments may matter, too. One study showed that the bees tend to be more common on irrigated farms. And another found that they seem willing to dig through a modest, 2-inch layer of mulch to make their nests—although these researchers had some trouble with their bees and only had four nests to go on.

Finally, be aware of pesticides in the garden. Squash plants get some pests—such as cucumber beetles and squash vine borers—that may require your intervention. But never apply an insecticide to a flowering squash plant, and bear in mind that systemic insecticides applied before flowering can still turn up at toxic levels in squash pollen. The immature bees in underground nest cells will be exposed to chemicals applied to the soil at any time of year, although no research has yet found a link between pesticide use and squash bee abundance on commercial farms.

Even if you’re not up early enough to see them, or even if you’ve had your harvest and torn out your squash vines—remember that these little bees are there year round, much of it spent underground waiting for the next season of squash flowers.

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As an insect ecologist, I am interested in plant-insect interactions and their responses to human modified environments. Plants and insects diversified together, producing the fascinating array of interactions we see in the world today—from seed dispersal and pollination to herbivory. My current research asks how urbanization and climate change alter plant-insect interactions, using scale insects and their host trees as a study system. I am comparing scale-insect abundance across urban, latitudinal, experimental, and historical temperature gradients. My other ongoing projects examine arthropod diversity and function across New York City green spaces, and the effects of Hurricane Sandy on New York’s urban insects. Ultimately, my goal is to understand how human activities, including both urbanization and restoration, can be guided to preserve a diversity of plants, insects, and their interactions.